In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
For application in bulk heterojunction (BHJ) organic photovoltaic (OPV) cells, a semiconductor is required that has a low band-gap to enable improved light harvesting by the photoactive layer that can lead to higher cell efficiencies.
Further requirements for the semiconductor are good solution processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
Nitrogen containing small molecules, oligomers and polymers have demonstrated interesting hole transport properties.1-5 Various materials have been developed to take advantage of this physical property in organic light emitting devices (OLED), in organic field-effect transistors (OFET) and organic photovoltaic cells (OPV). However, most of those materials show poor solubility or poor structural organization in the solid state.1-5 Furthermore, these materials have generally required complex synthetic routes to yield the final material.
Recently materials were reported that combine fluorene and phenanthrene core units, leading to highly thermally stable polymers derivatives, namely poly(4H-cyclopenta[def]phenanthrene), for use in OLED and OPV devices.6-8 However these materials are reported to show a low performance due to their low charge transport properties.
Therefore, there is still a need for OSC materials that do not have the drawbacks of prior art materials and are suitable for use in OFET and BHJ OPV devices.
The materials should be easy to synthesize, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good solution processibilty i.e. a high solubility in organic solvents, and high stability in air. For use in OPV cells, they should have a low band-gap, which enables improved light harvesting by the photoactive layer and can lead to higher cell efficiencies. For use in OFETs there is also a need for OSC materials that allow improved charge injection into the semiconducting layer from the source-drain electrodes.
It was an aim of the present invention to provide improved polymers for use as OSC materials especially in BHJ OPV devices, but also in OFET devices, which show the above-mentioned advantageous properties, and which do not show the above-mentioned disadvantages of prior art materials. Another aim of the invention was to extend the pool of OSC materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing the copolymers as described hereinafter, which contain optionally substituted 8,9-dihydrobenzo[def]carbazole units.
The 8,9-dihydrobenzo[def]carbazole unit has not been yet integrated into oligomeric or polymeric structure. Polymers containing this unit are expected to show improved hole transport and photovoltaic properties, compared e.g. to the already known poly(4H-cyclopenta[def]phenanthrenes). It is believed that the additional nitrogen atom in the structure has an influence on this improved behaviour by increasing the electron density within the unit.